Gaseous-discharge lamps such as mercury vapor and other metallic additive vapor lamps, have long been employed in industrial lighting situations because of their extremely high efficiency when compared with other sources, such as incandescent lights. Many of the high pressure gaseous-discharge lamps, such as, for example, the mercury vapor lamp, are ignited by applying a voltage across a starting electrode and one of the main electrodes of the lamp to ionize some of the gas in the lamp arc tube. However, some of the more efficient and newer high pressure lamps, such as, for example, some metallic additive and the high pressure sodium lamps, do not readily permit the use of a starting electrode. To start such lamps, a plurality of high voltage pulses must be applied to ionize the gas in the arc tube. Typically, such a lamp may require a minimum of fifty pulses per second, where each pulse must have a peak voltage of more than 2,500 volts sustained for more than one microsecond.
Heretofore, circuits have been available which apply the necessary high voltage pulses to the lamp to ignite the lamp. However, some of these circuits, such as shown in British Pat. No. 1,119,874, continuously apply pulses each cycle or half-cycle to the lamp even after the lamp is lit. This continuously causes radio frequency interference as well as continuously stressing the components of the circuit by the application of the high voltage pulses thereto. Other circuits, which are designed to discontinue the application of the starting pulses once the lamp is lit, do so by detecting a current flow in the lamp side of the circuit, which is the portion of the circuit subjected to the high voltage pulses. Once such circuit, shown in Canadian Pat. No. 826,768, has a thermal switch which is opened when the lamp ignites to discontinue the application of high voltage pulses. One of the disadvantages of such a circuit is that power is continually dissipated through the thermal switch when the lamp is on. Another circuit, is the type shown in U.S. Pat. No. 3,407,334 wherein components for discontinuing the high voltage pulses when the lamp ignites are connected to the lamp side of the circuit. Consequently, these components are subjected to high voltage pulses whenever the lamp is not lit. The significant drawback of such a circuit is that when the lamp is extinguished for a long period, such as by failure of the lamp, high voltage pulses are continuously applied to the control components. Normally, lamps in any particular installation may not be checked and replaced for days or weeks at a time. Furthermore, since in normal operation the control components are only momentarily subjected to high voltage pulses before ignition of the lamp, the values of these components are generally selected on the basis of the normal operating voltages which are applied thereto. To select components having high voltage ratings would undesirably increase both the physical size of the circuitry as well as the cost. Thus, in such a circuit, when a lamp is not replaced soon after failure, the control components are subjected to the continuous application of perhaps millions of high voltage pulses. This almost invariably results in the failure of one or more of these components. Moreover, the lamp ballast and control circuit components are normally an integral package which, upon failure of any one component, must be completely replaced. It is not only expensive to replace such components, but it is also time consuming and laborious to do so, especially considering that most lamp fixtures are placed in relatively high places which are difficult to reach.